you could go with an inline (if you have a canister) or use 2x 200W plus heaters if you have a sump or just place them into the tank... 40s is cold and its all dependent on what you what to house in said aquarium and what temp range you are shooting for. In my experience, inline heaters seem to be more effective at holding temps. Remember, you often pay for what you get.
Another idea would be to use a glass lid to help hold on the heat
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